This thread still has a bunch of conflicting info in it. I am confused.

Does the ACD ever allow the rear to free wheel from the front meaning the car could be 0/100 or FWD?

Is the split mechanical or is it hydraulic?

If the split is always 50/50 like it is on the 03/04 how can it provide more torque to wheels requiring torque? That would mean the distribution is no longer 50/50.

We know power isn't being balanced left to right only front to rear. Left to right is controlled by the mechanical LSD inside the front and rear diff. So if the center diff is active that must mean that it can shift the balance of power front to rear depending upon what the ECU thinks needs more power.

 

I think a bacic concept that has some people confused is the difference between torque split and wheel speed. I will try to explain. Hopefully I can get my thoughts across into words that make sense

First we will make the assumption that the center differential works just like a front or rear differential. You have one input (engine power) that is split into two outputs (front and rear, or left and right).

• Let's look at the most basic setup, a RWD car with an open diff (50:50 torque split). We place the left tire on ice, and the right tire on dry pavement. With an open differential, when you hit the gas the tire on the ice will spin faster than the tire on the pavement. The tires both receive the same torque levels, but the one on ice has less grip so it spins faster. You have 50:50 torque split but have wheelspin. In this case let's say you can put down 100#ft of torque, 50#ft goes to each tire.
• Let's do the same experiment with one side on dirt and one side on pavement. The dirt has a little more grip than the ice, so you will be able to put down more torque and accelerate faster. In this case you may put down 200#ft of torque, 100#ft to each tire.
• Now we will do the same experiment with a welded/spooled/locked rear diff. In this case the wheel speeds will be identical, but all of the torque will be applied through the tire with the most traction. In this case you may put down 300#ft of torque, all of which will be going to the tire on the pavement.
• A limited slip differential is a compromise. You give up raw straight-line traction of a "locked" diff, and gain driveability of an "open" diff. In our above experiment on dirt and pavement - the LSD clutches or viscous unit send more torque to the tire with more traction, but some torque will still make it to the tire on the dirt and it may spin a little faster than the other, but not nearly as bad as with an open diff. In this scenario, depending on the action of the diff, you may be able to put down 250#ft of torque, 200 to the pavement, 50 to the dirt. The limit of how much torque can be trasferred is defined by the makeup of the diff, design, components, spring rates, viscosity coefficients, etc. . .

Now applying this to the EVO center differential, specifically the ACD version:

• The actual Center Differential is the gear mechanism inside the transmission that takes the power input from the engine (pinion gear turns the ring gear) and spits it into two shafts that basically lead to the front and the rear ends. The center differential unit is an open differential, with a 50:50 torque split, just like the rear diffs discussed in the first 2 points above.
• The VCU or the ACD unit, depending on model year, is what is responsible for adjusting and moving the torque split.
• With the ACD "open" or disengaged, F/R torque is split 50/50, and wheel speeds vary via slippage to keep it this way. When the ACD "closes" or engages due to excessive slippage, wheel speeds are held closer to 50/50, while torque is trasferred to the wheels with more grip.
• When you pull the E-brake, you disengage the ACD, and the diff acts once again as an open diff.
• When you pull the E-brake on a VCU equipped car, there is no way to "disengage" the VCU, so you end up sending braking forces from the rear tires, through the driveshaft, into the transfercase (putting reverse force on the gears), into the VCU, which connects to the center diff, and forces the front tires to brake also. That is why is is bad to Ebrake in VCU equipped EVO's - puts a huge strain on the VCU and transfer case. Basically like sticking your car in reverse and trying to do a burnout. Your TC gears were not designed for those forces. (I learned the hard way - Thanks TRE!! )

OK, so now hopefully you can see where alot of this confusion is coming from. It is all about semantics. What exactly to people mean when they say "center differential". If they mean the just the differential gears in the tranny, then YES, it is full time 50:50. If they are also including the whole VCU or ACU unit as part of the greater center diff assembly, then NO, it is not 50:50.

This is why Mitsu says the center diff has 50:50 torque split, and Motec says their ACD controller can adjust torque split. They are both correct. . .

EVOlutionary

 

Beat me to the punch.

Like he said, if you cant understand the 50/50 issue, read up on how a differential works.

Terry S

 

Here's the thing.....50/50 means just that you cannot change the torque split without changing the 50/50. If you had an open diff and one tire was on ice all the torque goes to that tire, the other wheel wont do ****, I've been there.

Explain to me how you can get more power to the rear without changing the 50/50 split. The only thing I can tell it might do is apply pressure to the freewheeling tire to transfer more power to the other. For what its worth this is not the only thread about ACD and all end up in the same spot.

I'm well aware how the center diff works. I had a spool in my GSX that was a true locked diff. I guess what I am really looking for is an explanation of what OPEN means to ACD.

 

"If you had an open diff and one tire was on ice all the torque goes to that tire, the other wheel wont do ****"

Not true. That is what I tried to explain with that long post. There is a difference between torque applied to the road and wheel speed. If you were in this situation and you floored it, the tire on ice would spin wildly while the other tire would not spin any faster than the 5 or 10 mph you may be going. . . . but BOTH tires will be putting down the same small amount of torque to the ground. They may both be driving the car forward slowly with 50#ft of torque each, but because the ice has lower friction, that tire spins faster to apply the 50#ft of torque.

"Explain to me how you can get more power to the rear without changing the 50/50 split."

You can't at a set engine power output - you have to change the 50:50 split. I explained it in my above post. Here is is again. The actual differential gears have a 50:50 distribution. The ACD unit or the VCU unit is a separate (yet connected) mechanism, and it is this mechanism that changes the torque split by "locking" to various degrees the two sides of the differential gear together (like your spool).

Open means exactly that, it acts as an open diff. 50:50 split.

EVOlutionary

 

Exactly.....

ACD changes the split to something other than 50/50.

 

OK... lets approach this barney style.

Click on this link so we are looking at the same thing:

http://static.howstuffworks.com/flash/differential.swf

If you notice, this is an open differential. And if you notice, because the number of teeth in the gears for each pinion and wheel output shaft is the same, that indicates a 50/50 split (same number = same split).

Now click on the turn button. If you'll notice, the output gears are spinning at different speeds. The torque hasn't changed. The number of teeth are still the same so the same amount of power is going to each output. BUT one side is spinning faster than the other. This is because the traction of that side is less than the other. Like we discribed earlier with the ice thing.

What the ACD does is *prevents* this difference in speed between the front set and rear set of wheels to a certain degree. When the ACD is in full locked at full strength, it can prevent it up to 2000Nm of force. If the amount of torque is beyond that amount, then the ACD cannot prevent the amount over 2000.

One thing to remember too is that the ACD is NOT an on/off switch. It is more like a light dimmer. Different settings and different conditions means the ACD will "clamp down" in differing amounts.

For instance, in Tarmac it will clamp down with 100% of the force when accelerating in a straight line. Also, it will release the clamping almost immediately upon detecting the car turning.

In the snow mode, the car clamps down with less than 100% of the force (not sure the actual %) when accelerating in a straight line. Also, it will retain the clamping down for several seconds into a turn unlike the tarmac mode.

Hopefully this info helps to clear up your questions.

Terry S

 

The split changes....its not 50/50 once ACD starts to affect it. The flash page is great but it says nothing about power/torque applied to each side and we aren't really talking about left to right. What the ACD does is electronically allow one diff to get more power than the other diff. It acts in different ways in different settings only by how long it takes to kick in and out. But it definately uses hydraulic pressure to give the front or rear more power to the wheels.

 

No.

Again.

It's just a differential.

It stops one side from spinning faster than the other when engaged, it allows it when disengaged.

Thats it.

Terry S

 

Terry, is there any slip from front to back (difference between the average left and right rotation rate each for front and back)?

If not, then I guess a mild overall tire circumference difference between the front and rear tires can be bad. I found that just a 5psi difference in tires can noticably change the rotation rate

 

It controls front to rear transfer only. Left to right is controlled by the F/R LSD.

 

So what if I ask this way:

I am at a dead stop with my front wheels on ice, and my back wheels on dry pavement. I floor the throttle.

- with an open center diff. the front wheels spin like crazy, and the rear wheels do not rotate. and he car does not move.

- with a closed diff my back tires will receive some torque, regardless of the front wheels spinning freely, and the car moves forward, but not as quickly as if all 4 wheels were on dry pavement.

right?


but the front wheels are not exerting any torque on the axles, since they are just spinning on frictionless ice, while the rear wheels are exerting torque onto the axles, since those tires have some traction.


I guess what I'd prefer is the system they had in the 05 STi, where you could send more torque to the rear wheels than the front, and get some of the handling characteristics of a rear wheel drive car if you wanted.

 

On my non-ACD car the front is directly linked to the back. When my car is jacked up at all four corners and I spin any one wheel all 4 wheels spin at that speed. Whether the ebrake is on or not.

 

Scenario 1, correct, both are getting 50:50 torque distribution, which in this case is near ZERO.

Scenario 2, correct, the "limited slip" or "locking" action of the ACD transfers more torque to the rear wheels and you move forward at a close to normal pace (assuming your rear tires stay on pavement and do not spin from excessive power).

This is another example of how the center differential GEARS are mechanically a 50/50 split, but the ACD unit attached to it can alter/move the torque split. . .

EVOlutionary

 

So with your E-Brake on you can spin your tires by hand? Might want to adjust it a bit . . .